This invention relates generally to the provision of real-time services over a packet network, and more particularly to the provision of Internet telephony over a CATV cable network.
Today, access to the Internet is available to a wide audience through the public switched telephone network (PSTN). Typically, in this environment, a user accesses the Internet though a full-duplex dial-up connection through a PSTN modem, which may offer data rates as high as 56 thousand bits per second (56 kbps) over the local-loop plant.
However, in order to increase data rates (and therefore improve response time), other data services are either being offered to the public, or are being planned, such as data communications using full-duplex cable television (CATV) modems, which offer a significantly higher data rate over the CATV plant than the above-mentioned PSTN-based modem. Services under consideration by cable operators include packet telephony service, videoconference service, T1/frame relay equivalent service, and many others.
Various standards have been proposed to allow transparent bi-directional transfer of Internet Protocol (IP) traffic between the cable system headend and customer locations over an all-coaxial or hybrid-fiber/coax (HFC) cable network. One such standard, which has been developed by the Cable Television Laboratories, is referred to as Interim Specification DOCSIS 1.1. Among other things, DOCSIS 1.1 specifies a scheme for service flow for real-time services such as packet telephony.
When real-time services are offered over a packet-switched network such as the Internet the delay in transmission between the time the packet is generated by a user at one end point and the time it is received at another endpoint can be problematic. This is particularly true if the real-time service being offered is packet telephony. Individuals can endure upwards of approximately 250 milliseconds of delay during a voice conversation before the conversation becomes awkward. One contribution to the delay is due to the time interval between the time a data packet is generated by the end user and the time it is actually transmitted to the cable system headend. This time interval, which can contribute about 20 milliseconds of delay, arises because of a lack of synchronization between the time when the data is ready to be transmitted and the time when the authorization to transmit is received
Accordingly, it would be advantageous to reduce the end-to-end delay when providing real-time packet-switched service over a cable network.
The present invention provides a method for use in providing real-time, packet-switched service to an end-user over a cable data network. The method begins by transmitting over the cable data network a scheduled grant to an end-user gateway in accordance with an unsolicited grant service protocol. The grant authorizes the end-user gateway to transmit a data packet to a cable modem termination system (CMTS) located in the cable data network. Next, an adjustment is made to the time at which subsequent grants are transmitted. The adjustment, which is based on a response of the end-user gateway, is performed to reduce delay between a time when a subsequent data packet is generated and receipt of the subsequent grant.
In accordance with one aspect of the invention, at least one additional grant, or one pair of additional grants, is transmitted in addition to scheduled grants to reduce the delay.
In accordance with another aspect of the invention, the scheduled grants are transmitted at a nominal grant time and the additional grant is transmitted at a time before the nominal grant time.
In accordance with yet another aspect of the invention, the CMTS receives a data packet in response to the additional grant and another additional grant is transmitted at a time before the initial additional grant is transmitted.
In accordance with another aspect of the invention, the CMTS receives a data packet in response to the additional grant. The data packet includes a queue indicator bit requesting another grant. In addition, a dribble grant is transmitted in response to the queue indicator bit.
In accordance with another aspect of the invention, a cable modem termination system is provided for use in providing real-time packet-switched service to an end-user over a cable data network. The system includes an apparatus for transmitting over the cable data network a scheduled grant to an end-user gateway in accordance with an unsolicited grant service protocol. The grant authorizes the end-user gateway to transmit a data packet to a cable modem termination system (CMTS) in the cable data network. The system also includes an apparatus for adjusting a transmission time of subsequent grants, based on a response of the end-user gateway, to reduce delay between a time when a subsequent data packet is generated and receipt of the subsequent grant.